76 ❯ STEP 4. Review the Knowledge You Need to Score High
Random assignment—division of the sample into groups so that every individual has
an equal chance of being put in any group or condition.Confounding variables—factors that cause differences between the experimental group
and the control group other than the independent variable.Operational definition—a description of the specific procedure used to determine the
presence of a variable.Experimenter bias—a phenomenon that occurs when a researcher’s expectations or
preferences about the outcome of a study influence the results obtained.Demand characteristics—clues participants discover about the purpose of the study
that suggest how they should respond.Single-blind procedure—research design in which participants don’t know whether
they are in the experimental or control group.Double-blind procedure—research design in which neither the experimenter nor the
participants know who is in the experimental group and who is in the control group.Placebo—a physical or psychological treatment given to the control group that resem-
bles the treatment given to the experimental group, but that contains no active ingredient.Placebo effect—a response to the belief that the independent variable will have an
effect, rather than the actual effect of the independent variable, which can be a con-
founding variable.Reliability—consistency or repeatability of results.Validity—the extent to which an instrument measures or predicts what it is supposed
to measure or predict.Statistics—a field that involves the analysis of numerical data about representative
samples of populations.Descriptive statistics—numbers that summarize a set of research data obtained from a
sample. Key concepts of descriptive statistics include:• Frequency distribution—an orderly arrangement of scores indicating the frequency
of each score or group of scores.• Central tendency—average or most typical scores of a set of research data or dis-
tribution.
mode—most frequently occurring score in a set of research data (“quick and dirty”).
median—the middle score when a set of data is ordered by size.
mean—the arithmetic average of a set of scores.• Variability—the spread or dispersion of a set of research data or distribution.
• Range—the difference between the largest score and the smallest score (“quick and
dirty”).• Standard deviation (SD)—measures the average difference between each score and
the mean of the data set.• Normal distribution—bell-shaped curve that represents data about how lots of
human characteristics are dispersed in the population.